Clean intersecting geometry
This is a common problem. You're probably getting a &quot;heat weighting failed&quot; error, as described here:
heat weighting: failed to find solution for one or more bones.
This happens when you're trying to bind an armature to a mesh that consists of one or more pieces of non-manifold geometry. It doesn't matter whether these pieces have been joined together or not. If there is non-manifold geometry, you can encounter this issue. The only solution is to rebuild the mesh to get rid of anything non-manifold, so that you end up with one single, continuous mesh with no mesh collisions or holes. That will likely be a labor-intensive effort, but you might be able to reduce some of the workload by using the Mesh Cleanup tools.